Chlorohydrins optically active

The optically active glycols are a convenient starting material for the preparation of optically active carbinols, hydroxy-acids, etc. The biological method of asymmetric reduction is perhaps the only convenient method for the preparation of these glycols. The steps in the preparation of other optically active glycols arc identical with those of /-propylene glycol. In some cases it is found convenient to oxidize the chlorohydrin to the... 

Finally, mention may be made of those articles in which this method is utilized in the synthesis of optically active oxiranes for example, the simple synthesis of monosubstituted (S)-oxiranes and the asymmetric cyclization of some chlorohydrins catalyzed by optically active cobalt (salen)-type complexes, or in the enantiomeric selection of racemic oxiranes via halohydrins and /3-hydroxy sulfides. A useful three-step synthesis has been worked out from (S)-amino acids to (R)-alkyloxiranes as well as enantiomer resolution for chiral oxiranes by complexation gas chromatography. ... 

Chloroethanol is also formed from ethylene in the presence of pyridine and necessarily cupric chloride even with low chloride ion concentration [55], whereas in the absence of CUCI2 acetaldehyde is the sole product. With chiral amines and phosphines as ligands instead of pyridine, optically active chlorohydrins can be obtained from higher a-oleftns [56, 57]. 

Another example of the reduction of a-chloroketone involves dynamic kinetic resolution. The reduction of an a-chloroketo ester by M. racemosus and R. glutinis resulted in optically active syn- and anti-chlorohydrin, respectively, as shown in... 

Both (-)-aplysin (117) and (-)-debromoaplysin (116) have been synthesized (Scheme 8). The initial step involves the coupling of the chlorocyclopentenone (114) with the optically active metalated bromo-ether (113), which was derived from (-l-)-a-pinene. This reaction produces (115) together with the diastereoisomeric chlorohydrin. In another synthesis of aplysin (117) (Scheme 9) the key step is the acid-catalysed rearrangement of the trichothecane-type... 

Ethylene chlorohydrin (38) is formed in the Wacker process as a byproduct. Chlorohydrins are obtained as main products when PdCl3 (pyridine) is used [29]. The optically active chlorohydrin 40 with high ee was obtained without forming the regioisomer 41 from allyl phenyl ether (39) and other substituted alkenes when the bimetallic Pd complex 42 coordinated by water soluble chiral BINAP-based ligand 43 (II-9) was used [30]. 

Oxirans.—A simple four-stage preparation of (5)-propylene oxide from ethyl L-( —)-maleate has been described (Scheme 2). This work is of importance for the synthesis of nonactin carboxylic acid. Another synthesis of optically-active propylene oxide involves the cyclization of OL-propylene chlorohydrin with a variety of bases in the presence of a cobalt complex the highest optical purity was 27%. Wynberg and co-workers have shown that the base-catalysed epoxidation of electron-poor alkenes is subject to catalytic asymmetric induction hydrogen peroxide and t-butyl hydroperoxide were used as oxidants in the presence of quaternary... 

Umani-Ronchi adapted the Furstner protocol to achieve the first catalytic, enantioselective variant of this reaction. The chiral chromium salen complex was prepared from the in situ reduction of the anhydrous CrCb to CrCl2 with an excess of manganese metal, followed by complexation with the salen ligand 8 in the presence of catalytic triethylamine." Then the addition of allylic chloride (9) to aldehydes 10 to give the allylic alcohols 11 in moderate yields and in up to 95% ee. The same groups employed the same conditions for the addition of 2-butenyl bromides to aldehydes to achieve up to 83 17 syn/anti of allylic alcohol products and for the addition of 1,3-dichloropropene to aromatic aldehydes to obtain the syn chlorohydrin adduct in modest yield which were further converted to optically active vinyl epoxides. The [Cr(salen)]-catalyzed addition of propargyl halides to aromatic aldehydes allowed the synthesis of enantiomerically enriched homopropargyl alcohols in moderate yields with up to 56% ee. ... 

Optically active, vicinal chlorohydrins can serve as building blocks in much the same capacity as epoxides, azido alcohols, or diols. Enantioselec-tive access to chlorohydrins such as 165 was made possible through Henry s discoveiy of a Pd-catalyzed interrupted Wacker oxidation of olefins (Equation 28) [136, 137]. The process employs tetrasulfonated BINAP 166 as a chiral ligand embedded within the bimetallic triketone complex 164 [136]. 

In an effort to develop easy-to-use ketoreductase toolbox , we have surveyed the activity and enantioselectivity of a collection of ketoreductases (KRED) from various sources toward the reduction of a variety of ketones [90,91]. These studies served as a useful guideline for developing enzymatic processes for the production of optically pure chiral alcohols. For example, several chiral chlorohydrins of pharmaceutical importance were synthesized in both enantiomeric forms using the enzymes in this ketoreductase collection (Table 7.2) [92]. Further applications of this collection and other commercially available ketoreductases can be found in a recent review [9]. 

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